Many children with ADHD are able to succeed academically. They study in the usual schools and do not have to go to a special school. Very often they will benefit from treatment with medications and/or behavioural interventions. So it is important to obtain help from a mental health professional and do seek help early as early interventions augur a good outcome.
Around your child' education, your influence can be profound. If your child has persistent academic difficulties, do seek out a comprehensive evaluation to see if there are comorbid issues, especially if he has problems with reading or writing.
What you as a parent can do
You would need to work closely with the school to negotiate for academic accommodations at school. It is possible to have more time for the exams if the child has ADHD.
Sometimes, a prompter can also be useful to remind the person to move on to the next page of the exam script. At home, you can also help your child create a homework routine and help him/her stick to it.